Forget anything a dealer tells you that isn't written down in legalese.
Ask a pro tire guy what he thinks of that idea (mine cusses if I bring up the subject).
And Bumpyroad, I horselaugh at the people selling N for tires. "but the O will leak out and the N wont!" sez they. Well since we all start with 80% N anyway, why not just refill when that pesky O leaks out and youll have almost 100% N eventually anyway. Duh.
Science Matters.